1. Field of the Invention
This invention relates to magnetic disk apparatus, and more particularly to a new and improved magnetic disk control circuit.
2. Description of the Prior Art
A magnetic disk apparatus of the prior art is shown in FIG. 1. Disposed on a carriage 1 is a servo head 2 and a read/write head (not shown) to read and write data. A voice coil 3 drives the carriage 1 to transfer the servo head 2 and the read/write head. A preamplifier 4 amplifies servo data detected by the servo head 2 and provides the servo data for a transducer 8. A position formation circuit 5 sample-holds the servo data and outputs a position signal, indicating the position of the head, to a signal line 18. A track pulse formation circuit 7 forms a track pulse and outputs it to a signal line 19 to detect the level of the position signal. The formation of the track pulse is explained in more detail hereinafter. In FIG. 2, the position signal formation circuit 5 outputs the position signal (A) to transfer the servo head (T) on the magnetic disk (M) in the direction of the arrows. When the level of the position signal (A) is 0, the servo head (T) is positioned at the center data track (not shown). A track pulse B will be obtained each time the servo head crosses a data track, by a predetermined amount, which is detected when the level of the position signal A is either higher than the upper limit H or below the lower limit L. At no other time is a track pulse formed. A speed detecting circuit 6 includes a differential circuit which differentiates the position signal with respect to time in order to detect the transferring speed of the head and then to output a speed signal to a signal line 20 indicating the speed of the head at that time. A conroller 9 controls positioning of the head. When the controller 9 accepts a command to transfer the head from a magnetic disk control device (not shown), it calculates the number of tracks from the present position of the head to the target track, and sets the calculated number into a down counter 10. The down counter 10 counts down every time it receives a track pulse. Therefore, the down counter 10 always stores the number of tracks remaining between the track that the head is presently crossing and the target track in responding to a head transfer command.
A D-A converter 11 converts the count numbers, namely data for the number of tracks remaining until the target track is reached, into analog signals proportional to the number of tracks remaining. A distance-speed converter 13 is provided for the analog signals. Generally, there is a predetermined functional relation v=K.sqroot.l between the distance to the target track and the transfer speed of the head, in which the distance value is converted into a speed value wherein v is the transfer speed of the head, l is the number of tracks remaining until the target track is reached which is equal to the distance, and k is a constant. Therefore, the distance-speed converter 13 converts the level l of the analog signal provided by the D-A converter 11 into the level v of the target speed signal of the head and outputs it to a signal line 21 connected to one input of an analog different circuit 14. The target speed signal thereby indicates the instantaneous target transfer speed of the head. The difference circuit 14 has a second input connected to the speed signal from the speed detector 6 which corresponds to the actual speed of the head. The difference circuit 14 detects the voltage difference between the head target speed and the head actual speed and outputs the result to a power amplifier 17 via a signal selector circuit 16. The power amplifier 17 provides the voice coil 3 with a current corresponding to the voltage difference as an accelerating/decelerating signal and controls the transferring speed of the carriage 1.
When the head reaches the target track in a manner as explained above, the down counter 10 is decremented to 0 by the track pulse. At that time, a zero detecter 12 outputs a zero detecting signal via the output line 22 to the controller 9 signalling that the head has reached the target track. The signal selecter 16 is a gating circuit controlled by a control signal applied to a control input thereof from the controller 9 through the line 23. The controller 9 determines whether the position signal (A) at the output of the circuit 5 or the difference signal at the output of the difference circuit 14 will be effective in controlling the transfer speed of the carriage 1.
Most magnetic disk apparata generally have a track offset scheme to intentionally offset the position of the head to compensate for uses when the head misses or fails to stop at the proper track for various reasons. The offset voltage formation circuit 15 controlled by the control line 24 outputs a predetermined offset voltage to a signal line 25. The offset scheme adds the offset voltage to the position signal.
It is apparent that the above described prior art magnetic disk apparatus has a complicated structure to determine the target speed of the head by performing the function v=K.sqroot.l at the distant-speed converter. The prior art structure is further complicated by the necessity for the down counter, the zero detecter and the offset voltage formation circuit.
Reference is made to U.S. Pat. No. 4,068,269 and U.S. Pat. No. 4,237,502 for further background information, including circuit diagrams and flow charts, in the prior art.